Anatomy and Physiology: Drainage System
As you saw in The Blood, as well as earlier in this section, the plasma in the blood vessels leaves the capillaries, becomes interstitial fluid, and then is drained into lymphatic capillaries, where it is called lymph; lymph is ultimately drained back into the bloodstream via the subclavian veins. This constant movement of blood out of the capillaries has two potential problems: a drop in blood pressure as a result of the loss of blood volume, and a buildup of fluid in the peripheral tissues, known as edema. Yet, the role of the lymphatic system is far more complex than just drainage, as we shall see.
Lymphatics and Lymph Nodes
Two things differentiate lymphatic vessels from blood vessels: structure and direction. Although there are a surprising number of openings in capillary walls, the gaps in the endothelium of lymphatics are especially large. Unlike cardiovascular capillaries, which are continuous with arterioles and venules, lymphatics have blunt ends in the tissues. Those blunt ends illustrate perhaps the most important difference, which is that fluid in the lymphatics travels in only one direction: back to the heart.
Please understand that more than just fluid is being drained. Chemicals released into the interstitial fluid enter both types of capillaries. These chemicals include CO2 (to the lungs), wastes (to the kidneys), hormones (target organs all over the body), and so on. In this way, the lymphatic system is another delivery person. Some of those deliveries, such as bacteria or cancer cells, are not really wanted!
The Big Picture
One insidious way the body connects one system with another is due to the lymphatic system. Cancer cells have a habit of breaking away from tumors and traveling along lymphatics. They will divide when they reach the nodes, and they can ultimately spread to the entire body. Each of these new tumors, distant from the primary tumor, is called a metastasis. This is the main argument for early detection, for cancer is more likely to be cured if it can be treated before the malignant cells find their way on the lymphatic highway.
Have you ever stepped on a tack? AAAARRRGGHH!! A puncture wound—heck, even a little cut—introduces bacteria where it normally wouldn't be. The very outside layer of our skin is dry and dead (epithelial tissue is avascular, without blood nearby, the topmost layer will die;), which acts as a barrier to infection. Like any other living thing, bacteria need food, water, and warmth—sounds like a good description of the inside of your body.
Bacterial infections run the risk of infecting the whole body. The real risk is the introduction of bacteria into your bloodstream, which makes your blood septic, a condition known as septicemia; if untreated, this will kill you. The large gaps in the lymphatic endothelium are an easy route for bacteria. Since the lymph will be dumped into the bloodstream eventually, there had better be a way of getting rid of the bacteria first!
Luckily, there is! Lymph must first travel through lymph nodes (see Figure 12.6), and it is here that the main fight against infection happens. The vessels entering the nodes, called afferent vessels, and those leaving, called efferent vessels, have valves (see The Heart) to prevent backflow; indeed, these valves are present in all lymphatic vessels, as the lymph is at extremely low pressure, just like the veins, and the contents are pumped back the same way.
Given how they save our lives every day, it's amazing how small lymph nodes are (1-25 mm or up to an inch). There is an outer connective tissue capsule with walls called trabeculae that extend inside the node. In between these walls are pyramid-shaped areas, the wider portion of which is the cortex, and the narrower part is the medulla. Within each pyramid is a lymphatic nodule or follicle with those antibody factories, the B lymphocytes, inside, and T lymphocytes and macrophages on the outer edge. Together, these cells work to wipe out the infection before it spreads.
Cisterna Chyli, Thoracic Duct, Return
Flex Your Muscles
Don't assume that everything is split down the middle. The lymphatic system is one that doesn't follow a simple bilateral plan. The entire lower part of the body and the left arm and left side of the head and chest are drained into the left subclavian vein, but only the right arm and the right side of the head and chest drain into the right subclavian vein. That's not exactly even, but the job gets done.
Lymph nodes are both superficial and deep. Beyond that, and in addition to nodes scattered throughout the body, nodes can be found in larger groups that follow the lymphatics: inguinal nodes (in the groin), iliac nodes (along the external iliacs—remember the idea of parallel pathways?), intestinal nodes, axillary nodes (in the armpits), and cervical nodes. Lymphatics from the legs, the intestinal mesentery (see The Digestive System), and other abdominal regions, all collect in a dilated vessel beneath the diaphragm called the cisterna chyli (see Figure 12.7).
From the cisterna chyli there is a large duct that travels next to the spinal column, up the left side of the thoracic cavity. Name it. Oh, come on, you know it has to be called the thoracic duct! This duct is also called the left lymphatic duct because there is a much smaller, much shorter duct on the right hand side (the right lymphatic duct). These ducts merge with ducts from the arms (the left and right subclavian trunks) and the head (the left and right jugular trunks), and dump into each of the subclavian veins. And with that, the fluid, and all its contents (minus bacteria and viruses, thanks to those wonderful nodes) make their way back to the bloodstream.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.